A multiple-input multiple-output (Multiple Input Multiple Output, MIMO) technology is one of effective measures for enhancing spectrum efficiency of a radio communications system. For frequency division duplexing (Frequency Division Duplexing, FDD), precoding is generally combined with a limited feedback technology to obtain a transmitting array gain and implement inter-stream interference suppression, so as to achieve objectives of space multiplexing and improving a signal-to-noise ratio of received signals.
A precoding matrix or vector is obtained by a receiving side by measuring a common pilot or a measurement pilot, and is then fed back to a transmitting side by using signaling. The transmitting side performs corresponding MIMO transmission according to the precoding matrix or vector that is fed back.
For a current long term evolution (Long Term Evolution, LTE) system, a terminal may perform measurement by using a channel state information reference signal (Channel State Information reference signals, CSI-RS) and a cell-specific reference signal (Cell-specific Reference Signals, CRS). In an LTE FDD R8 (Release 8) protocol version, the terminal performs relevant precoding matrix indicator (Precoding Matrix Indicator, PMI) measurement by using a CRS. In a protocol of a version later than R9 (Release 9), a demodulation reference signal (Demodulation Reference Signal, DMRS) and a CSI-RS are introduced. A DMRS pilot is a user equipment (User Equipment, UE)-specific demodulation pilot. In both precoding and beamforming (Beamforming), a transmitting weight is borne by the DMRS pilot. A CSI-RS pilot is a measurement pilot, and is used for a UE to perform downlink channel measurement, including PMI measurement and channel quality indicator (Channel Quality Indicator, CQI) measurement.
As described in the LTE protocol, all CSI-RS-related parameters, such as the number of CSI-RS ports, a CSI-RS resource configuration sequence number, and a CSI-RS subframe configuration, can be delivered to the UE by using higher-layer signaling, which means that a CSI-RS may undergo user-level configuration. However, considering a limitation imposed by the protocol onto a configuration format of the CSI-RS, the CSI-RS is actually also a cell-level configuration. A main purpose of introducing higher-layer signaling to implement user-level CSI-RS configuration is to distinguish between measurement behaviors of UEs that support different protocol versions. For example, if a terminal that supports R8 and a terminal that supports R10 exist in a network, the terminal that supports R8 needs only to perform measurement on the CRS pilot, and does not need to perfoim CSI-RS configuration; but for the terminal that supports R10, with an increase in the number of transmitting antennas of a system, system overhead can be reduced by introducing the CSI-RS pilot, and therefore, for such type of UE, it is necessary to enable CSI-RS configuration and transmission.
Compared with an existing conventional macro site system, the foregoing technologies are a set of relatively complete solution. However, with continuous evolution of the LTE system, application of CSI-RS-related technologies similar to the foregoing may cause quite a number of additional problems to systems such as a heterogeneous network (Hetnet), a coordinated multi-point transmission (Coordinative Multiple-Point, CoMP) system, and a distributed antenna system (Distributed Antenna System, DAS).
For a CoMP system, if dependent JT transmission of multi-cell coordination needs to be implemented for a user at a cell edge, two coordinated cells need to implement user-level regular adjustment to implement signal synchronization between coordinated ports, and in addition, the UE needs to feed back phase difference information of signals of the two cells, which means a significant increase in system feedback overhead. Therefore, in a case where the phase difference information is unavailable, only independent JT transmission can be implemented.
In the current conventional MIMO technology, with given specifications of the transmitting antenna, the system determines the number of transmitting ranks (the number of ranks) according to the number of receiving antennas of the UE and a current channel state. For data at each rank, all antennas at a transmitting end participate in transmission. In a conventional MIMO system, the number of transmitting antennas visible to the UE is fixed, and all UEs in the system have the same number of transmitting antennas. After the number of transmitting antennas is determined, the system may deliver the CSI-RS pilot configuration to the UE by using higher-layer signaling. The UE may find, according to its own CSI-RS configuration, a corresponding pilot location to perform downlink channel measurement, and feed back information that is obtained by means of measurement, such as the PMI and CQI, to the transmitting side. The transmitting side selects a corresponding precoding matrix according to the PMI information fed back, and transmits weight information together with data signals to the UE by using a DMRS pilot. The UE performs channel estimation according to the DMRS pilot, so as to obtain the weight information for data demodulation. In addition, the transmitting end may also adjust a modulation manner and the number of ranks according to the information fed back by the UE, such as the CQI, so as to better match the current channel state.
The R10 protocol provides a CSI-RS configuration method and a resource mapping manner, and, under a condition of a fixed number of transmitting antennas, provides various cell-level CSI-RS configuration and resource mapping manners. However, to implement coordinated transmission in scenarios such as a distributed small-cell base station system, a remote antenna, or multiple virtual sectors, for a terminal in a service area, the number of system-side antennas or ports required by a user in single-site transmission may be different from that required by a user in coordinated transmission, but the existing protocol standard does not provide a corresponding user-level CSI-RS configuration method for the CoMP technology in such scenarios.